For a long time, peptides were perceived as a poor drug candidates. They are prone to proteolytic degradation and rapid clearance in vivo through renal filtration. In addition, most of them are unable to maintain a well-defined three-dimensional structure, which is critical for their bioactivity, existing instead as a vast ensemble of conformational isomers (especially true for short, conformationally unrestricted analogues). Usually, they do not penetrate cells easily, hampering their use for intracellular targets. Over the years various strategies were employed to address these drawbacks, such as head-to-tail cyclization, lactam bridges, introduction of unusual-, α,α-disubstituted-, and (D)-amino acids, retro-inverso approach, lipidation/cholesterylation, multimerization, concatenation, development of targeted delivery systems, application of cell penetrating peptides (CPP), etc. Despite all shortcomings however, peptides possess advantages, chief among them is their practically unrestricted variability that can be tailored for specific needs. If relatively short, they may be also efficiently synthesized and purified using existing methodology. Due to various technological advances, new lead compounds may be relatively easy accessible through computer-aided rational design as well as randomization-based discovery methods utilizing solid-support-grafted peptide arrays and phage display. Recently, stapling has a prominent method in the development of peptide-drug candidates.
The peptide stapling strategy for stabilizing of peptide α-helices utilizes a ring-closing metathesis (RCM) reaction. The “staple” is efficiently created in a two-step process between strategically positioned olefin functionalized non-natural amino acid side chains. The first step, catalyzed by Grubbs catalyst, results in olefin containing bridge that is subsequently catalytically reduced to saturated hydrocarbon (alkane), effectively locking the peptide into a stable α-helix conformation. Such helix stabilization had been shown to dramatically increase the helicity, potency resistance to proteolytic degradation and cell permeability of α-helical peptides. However, the use of stapling technology is limited, especially in academic settings due to relatively high cost of olefin-containing amino acids.